Injection molding method

ABSTRACT

The invention provides an injection molding method comprising supplying a gas into a molten resin in a plasticizing cylinder and injecting the molten resin having the gas dissolved therein, wherein when the resin is being plasticized, a gas space with a predetermined gas pressure is formed within the plasticizing cylinder in a gas supply section and the pressure at the front end of the screw is adjusted to be at least equal to the gas pressure in the gas space and within a range where the gas space can be maintained within the plasticizing cylinder in the gas supply section. According to the invention, the necessary amount of gas can be dissolved in the molten resin with good reproducibility in a quantitative manner.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/JP01/04622 which has an Internationalfiling date of May 31, 2001, which designated the United States ofAmerica.

TECHNICAL FIELD

This invention relates to a method of injection molding a thermoplasticresin, more particularly, to a method of injection molding comprisinginjecting a molten resin having a gas dissolved therein in order toenhance the flowability of the molten resin upon being molded or toobtain a foamed molded article.

BACKGROUND ART

The demand for smaller thickness and lighter weight is not limited tothe housings of mobile electronic devices such as portable computers andcell phones and it has recently become intensive in the field of generalelectronic devices. In particular, the chassis and the internalmechanistic parts of a copying machine and the like are required to havenot only high dimensional precision and various kinds of strengthassociated with handling but also reduction in thickness and weight. Asa result, injection moldings are needed that desirably have nonuniformsections, i.e., the portions that do not require strength are made thinand lightweight whereas the portions that require strength are madethick, and which still have good dimensional precision. In other words,it is required to meet both requirements for strength and lightweight byproviding a design in which the portions that require strength arereinforced with thick ribs whereas the portions that do not requirestrength are made as thin as possible. Under these circumstances, amolding method is needed by which even the portions that are thin-walledand have long flow distances can be adequately filled with a resinduring molding.

In order to ensure that even the portions that are thin-walled and havelong flow distances can be adequately filled with a resin, one mayenhance the flowability of the resin. In injection molding ofthermoplastic resins, the flowability of a molten resin determines notonly the ease in filling the mold cavity but also the probability thatafter filling the cavity, sufficient pressure is transmitted to itsinterior, particularly to the resin which forms the thin-walled portionsat the end of resin flowing; hence, the flowability of a molten resinalso affects the dimensional precision of moldings and is an importantfactor that determines the processability of resins.

One index of flowability is the viscosity of molten resin. Thermoplasticresins have high melt viscosity and are poor in flowability as moldingmaterials. Hence, in the case of thin-walled parts, incomplete resinfilling often occurs.

In order to lower the viscosity of molten resin and thereby improve theflowability, it is effective to increase the molding temperature;however, in the case of a resin for which the molding temperature isclose to its decomposition temperature or a resin incorporatingadditives such as less heat-stable flame retardants, the resin itself orthe additives may undergo thermal decomposition and problems are likelyto occur as exemplified by the decrease in the strength of moldings, theformation of foreign matter due to the deteriorated resin, the stainingof the mold and discoloration. Yet another problem is delayed cooling ofthe resin in the mold which contributes to prolonging the molding cycletime.

The following methods are conventionally known to be capable ofimproving the flowability of molten resin without increasing the moldingtemperature.

(1) Method of reducing the molecular weight of the resin by lowering itsaverage molecular weight or broadening the molecular weightdistribution, particularly by increasing the low-molecular weightcomponent.

(2) Method of introducing a comonomer into the molecule.

(3) Method of adding a low-molecular weight oily substance such asmineral oil or a plasticizer such as a higher aliphatic acid ester.

(4) Method of dissolving carbon dioxide which acts as a plasticizer.

To further describe method (4), reference may be had to WO 98/52734which teaches that carbon dioxide dissolved in a resin works as aplasticizer for the resin to lower its glass transition temperature.

It is known to produce foamed molded articles using molten resins havinga gas such as carbon dioxide dissolved therein. For example, thespecification of U.S. Pat. No. 5,334,356 discloses a method in whichcarbon dioxide used as a blowing agent is supplied into a molten resinas it flows part of the way through an extruder, thereby molding a fineand highly expanded microcellular foam. The official gazettes ofJapanese Patent Publication No. 57213/1988 and Japanese Patent Laid-OpenNo. 34130/1999 have descriptions of an injection molding machine forfoam molding using a gas and according to their teachings, a blowing gasis supplied at a site which is part of the way through an extruder and amechanism for preventing resin back-flow of a is provided in both thefirst and second stages of the screw and, optionally, also at the gassupply valve.

However, method (1) mentioned above lowers impact strength and chemicalresistance although it increases flowability; method (2) lowers hotrigidity; and method (3) has problems such as the plasticizer loweringhot rigidity or being deposited on the mold to stain it during molding.Method (4) has the advantage of not causing the problems encountered inmethods (1)-(3), however, the desired improvement in flowability isdifficult to achieve if an insufficient amount of carbon dioxide isdissolved in the molten resin.

The following two methods may be employed to dissolve gases such ascarbon dioxide in the resin. In the first method, a resin in particulateor powder form is preliminarily placed in a carbon dioxide atmosphereand supplied into the molding machine after it has been allowed toabsorb carbon dioxide. The amount of carbon dioxide absorption isdetermined by the pressure of carbon dioxide, the temperature of thecarbon dioxide atmosphere and the time for which carbon dioxide isabsorbed. In the other method, carbon dioxide is supplied and dissolvedin the plasticized resin in the cylinder of the molding machine.

To supply a gas such as carbon dioxide into molten resin in the commonlyused in-line screw type and screw preplunger type injection moldingmachines in which the screw rotates intermittently, the gas supply pumphas to be operated in accordance with the amount of resin transfer whichvaries with time as the screw stops or starts rotating, so it isdifficult to ensure that the amount of the gas dissolved in the resin iscontrolled at constant level.

The present invention has been accomplished in the light of theaforementioned conventional problems. Accordingly, an object of thepresent invention is to provide an injection molding method whichcomprises supplying a gas into a plasticizing cylinder and injecting amolten resin having the gas dissolved therein, and in which aquantitative and economically required volume of a gas such as carbondioxide can be dissolved in the molten resin even if the screw rotatesintermittently in the plasticizing cylinder.

DISCLOSURE OF THE INVENTION

The present invention provides an injection molding method comprisingsupplying a gas into a molten resin in a plasticizing cylinder andinjecting the molten resin having the gas dissolved therein,characterized in that when the resin is being plasticized, a gas spacewith a predetermined gas pressure is formed within the plasticizingcylinder in a gas supply section and the pressure at the front end ofthe screw is adjusted to be at least equal to the gas pressure in thegas space and within a range where the gas space can be maintainedwithin the plasticizing cylinder in the gas supply section.

The above-described injection molding method of the invention includepreferred embodiments as follows:

The gas pressure in the gas space formed in the gas supply section isdetected and the pressure at the front end of the screw is controlled onthe basis of this gas pressure;

The molten resin is transferred in a starved state within the gas supplysection;

The pressure at the front end of the screw exerted during plasticizationis also retained during the screw shutdown period from the end ofplasticization to the start of injection;

The gas is carbon dioxide;

The plasticizing cylinder is equipped with a multi-stage type screw inwhich a stage comprising, in the following order from the resin supplysection side toward the front end side of the screw, a feed section, acompression section and a metering section is repeated a plurality oftimes in series, and the gas supply section is located within the feedsection of the front end side screw stage;

Notches are formed in the screw flights in the feed section of the frontend side screw stage, and the gas and the molten resin are kneaded whilepart of the molten resin is caused to flow backward by means of saidnotches;

A flow control section presenting high resistance to the flow of themolten resin is provided in the metering section of the rear end sidescrew stage of the plasticizing cylinder;

The plasticizing cylinder is equipped with a mechanism for preventingthe back-flow of the molten resin in the metering section of the rearend side screw stage;

The gas supply into the plasticizing cylinder in the gas supply sectionis effected via an automatic on-off valve which is opened and closedautomatically by the pressure difference between the supply pressure andthe pressure within the cylinder in the gas supply section; and

An in-line screw type injection molding machine is used that is equippedwith a valve nozzle having a closable injection port and a mechanismprovided at the front end of the screw to prevent back-flow of themolten resin, wherein the screw is rotated with the injection port beingclosed, and as soon as a predetermined amount of the molten resin withdissolved carbon dioxide is accumulated in the front part of theplasticizing cylinder, the resin back-flow preventing mechanism in thefront end portion of the screw is brought to closure with the backpressure of the screw being maintained near the position where the screwcompletes metering, and then the injection port is opened to inject themolten resin.

The present invention also provides an injection molding apparatus whichcomprises a plasticizing cylinder and a gas supply device for supplyinga gas into a molten resin in the plasticizing cylinder and which injectsthe molten resin having the gas dissolved therein,

wherein said plasticizing cylinder has a multi-stage type screw in whicha stage comprising, in the following order from the rear end side towardthe front end side in the direction of injection, a feed section, acompression section and a metering section is repeated a plurality oftimes in series, and which is also equipped with a gas supply channelopen to the feed section of the front end side screw stage, and

wherein said supply device is connected to the gas supply channel.

The injection molding apparatus of the invention include preferredembodiments as follows:

The apparatus has a pressure sensor for detecting the pressure of thegas supplied into the plasticizing cylinder via the gas supply channeland a control unit for controlling the pressure at the front end of thescrew on the basis of the pressure as detected with this sensor;

The apparatus has an automatic on-off valve, which is pressed by aspring onto a valve seat provided on the perimeter of an opening in thegas supply channel that extends into the plasticizing cylinder andwhich, when gas is supplied into the gas supply channel, is pressedagainst the spring by the pressure difference between the supplypressure and the pressure within the cylinder in the gas supply sectionand moved into the plasticizing cylinder to open the gas supply channel;

The screw flights in the feed section of the front end side screw stagecontain notches; and

A flow control section presenting high resistance to the flow of themolten resin is provided in the metering section of the rear end sidescrew stage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an injection molding apparatus suitablefor the injection molding method of the invention;

FIG. 2 is a diagram showing details of a gas supply device comprisingthe carbon dioxide source, the carbon dioxide booster and the carbondioxide pressure control that are shown in FIG. 1 and a plasticizingcylinder portion;

FIG. 3 shows in enlarged section an example of an area of theplasticizing cylinder near the gas supply section;

FIG. 4 shows in section an automatic on-off valve mechanism in the gassupply section;

FIG. 5 is a graph showing the relationship between the pressure ofcarbon dioxide in the gas supply section and the amount of carbondioxide dissolved in molten resin;

FIG. 6 is a graph showing the relationship between the pressure at thefront end of a screw and the pressure of the molten resin in the gassupply section;

FIG. 7 is a diagram for illustrating the first and second stages of ascrew;

FIGS. 8(a) and 8(b) are diagrams showing an example of a notched screw;

FIG. 9 shows in enlarged section a back-flow preventing mechanismprovided at the front end of a screw;

FIG. 10 is a graph showing the relationship between the molding cycletime and the amount of carbon dioxide absorbed in Example 3;

FIG. 11 is a graph showing the relationship between the molding cycletime and the amount of carbon dioxide absorbed in Example 4.

In the drawings, respective numerals represent the following: 1,injection molding machine; 2, plasticizing cylinder; 3, mold; 4, moldclamping device; 5, carbon dioxide source; 6, carbon dioxide booster; 7,carbon dioxide pressure control; 8, hopper; 9, liquefied carbon dioxidecontainer; 10, electromagnetic on-off valve; 11, liquefied carbondioxide compressor; 12, electromagnetic on-off valve; 13, heater; 14,reducing valve; 15, main tank; 16, relief valve; 17, meter; 18, gassupply pipe; 19, electromagnetic on-off valve; 20, check valve; 21,relief valve; 22, valve open to the atmosphere; 23, screw; 23 a, firststage of the screw; 23 b, second stage of the screw; 24, nozzle portion;25, resin metering device; 26, gas supply section; 27, gas supplychannel; 28, flow control section; 29, automatic on-off valve; 30,spring; 31, valve seat; 32, valve shaft; 33, back-flow preventing ring;34, nozzle hole; 35, needle valve; 36, drive unit; 37, drive rod; 40,back-flow preventing mechanism; 41, first feed section; 42, firstcompression section; 43, first metering section; 44, second feedsection; 45, second compression section; 46, second metering section;50, notch; 60, small-diameter portion; 61, resin flow channel portion;62, check ring; 63, spring.

BEST MODE FOR CARRYING OUT THE INVENTION

The resins to be used in the injection molding method of the inventionare thermoplastic resins and specific examples include polyethylene,polypropylene, poly(vinyl chloride), acrylic resins, styrenic resins,poly(ethylene terephthalate), poly(butylene terephthalate),polyallylates, poly(phenylene ether), modified poly(phenylene ether)resins, wholly aromatic polyesters, polyacetals, polycarbonates,amorphous polyolefinic resins, polyetherimide, polyethersulfone,polyamide-based resins, polysulfones, polyetheretherketone, andpolyetherketone. These may be used either alone or as a blend of two ormore species They may also be used in the presence of variousincorporated fillers and additives.

Styrenic resins among the above-listed thermoplastic resins arehomopolymers and copolymers that use styrene as an essential ingredient,as well as polymer blends prepared from these polymers and other resins;polystyrene and ABS resin are preferred. Polystyrene is a styrenehomopolymer or a rubber-reinforced polystyrene having rubber distributedin the resin phase.

If carbon dioxide is to be used as gas, preferred thermoplastic resinsare those which have high enough affinity for carbon dioxide to bedissolved in large amount, as well as those which can be effectivelyplasticized with carbon dioxide; particularly preferred arepolyethylene, polypropylene, styrenic resins, polyacetals,polycarbonates, poly(phenylene ether) and modified poly(phenylene ether)resins. In particular, polycarbonates not only permit carbon dioxide tobe dissolved in large amount, they also generate carbon dioxide whendecomposed thermally; hence, the inclusion of carbon dioxide in themolten resin offers the added advantage of shifting the equilibrium ofthe decomposition reaction to slow down its rate.

The injection molding method of the invention is applicable to theproduction of general injection moldings and can be used to produce notonly foamed molded articles but also solid shapes having no cells in theinterior. The molten resin having gas dissolved therein has a tendencyto expand, so in order to prevent the development of a foam pattern onthe surface of the molding, the injection molding method of theinvention may be combined with the counterpressure method in which themold cavity is preliminarily pressurized with gas before it is filledwith the molten resin so that no foaming occurs in the flow front of themolten resin during cavity filling.

The gas to be used in the injection molding method of the invention maybe exemplified by nitrogen which is commonly used as a blowing gas,inert gases typified by rare gases such as helium and argon, carbondioxide which is highly soluble in thermoplastic resins and exhibits asatisfactory plasticizer effect, Freons comprising C₁-C₅ saturatedhydrocarbons in which some hydrogen atoms are replaced by fluorine, andthe vapors of liquids such as water and alcohols.

If gas is to be dissolved in order to enhance the flowability of moltenresin, carbon dioxide is the most preferred gas. Carbon dioxide has highsolubility in molten resin and meets various requirements such as nodeterioration of the resin, mold and the constituent material of themolding machine, no hazard to the environment in which molding isperformed, and low cost; in addition, if carbon dioxide is used as aplasticizer, it can be rapidly evaporated away from the molded articleafter molding. If gas is used as a blowing agent, nitrogen having highblowing action is preferred.

If carbon dioxide is used as a plasticizer in the injection moldingmethod of the invention, it is preferably dissolved in the molten resinin an amount of at least 0.2 wt %. There is no particular limitation onthe maximum amount by which carbon dioxide can be dissolved; however,high gas pressure is needed in order to dissolve a large amount ofcarbon dioxide and an undue increase in the amount of dissolution simplyreduces the effectiveness of carbon dioxide in improving the flowabilityof the resin; hence, a practical amount of carbon dioxide that may bedissolved is 10 wt % or less, more preferably 5 wt % or less.

In this connection, the amount of gas such as carbon dioxide that hasdissolved within the plasticizing cylinder is difficult to measuredirectly, so the difference between the weight of a molded articlemeasured immediately after injection molding of a carbon dioxidecontaining resin and the weight of the molded article measured afterleaving it to stand for at least 24 hours in a hot-air dryer at atemperature about 30° C. lower than the glass transition temperature ofthe resin if it is amorphous or the melting point of the resin if it iscrystalline, until the amount of the carbon dioxide contained in themolded article has leveled off as a result of dissipation is defined asthe amount of the carbon dioxide in the molten resin injected into themold cavity.

The injection molding machine to be used in the injection molding methodof the invention is exemplified by the in-line screw type injectionmolding machine and the screw preplunger type injection molding machinewhich are commonly used in injection molding and in which the screwrotates intermittently to plasticate the resin. In the in-line screwtype injection molding machine, the screw also serves as an injectionplunger and with the progress of resin plasticization, the screwretracts to shorten the effective screw length. In the screw preplungertype injection molding machine, the resin-plasticizing extruder portionis separate from the injection plunger, so the extruder portion can beregarded as a pure extruder. Hence, the screw preplunger type injectionmolding machine has fewer limiting factors in screw design and not onlyis it possible to increase L/D and decrease the root diameter, there isalso the advantage of providing ease in keeping gas to be supplied at anoptimum position.

In the injection molding method of the invention, gas is supplied intothe plasticizing cylinder and dissolved in the molten resin in theplasticizing cylinder. The molten resin with the gas dissolved thereinhas a tendency to expand, so in order to ensure that the pressure in theplasticizing cylinder will not escape from the nozzle portion of theinjection molding machine and that the molten resin with the dissolvedgas will not expand in the plasticizing cylinder, preparations forinjection are made with care being taken to prevent the internalpressure from escaping from the nozzle portion. To prevent the internalpressure from escaping from the nozzle portion, the plasticizingcylinder may have a valve nozzle equipped with a mechanism for openingand closing the nozzle hole. This mechanism may be exemplified by amoving needle that opens or closes the nozzle hole or a rotary valveprovided in the resin flow channel; a preferred structure opens orcloses near the nozzle hole in order to prevent drooling. If aninjection molding machine having no valve nozzle is to be used, theescape of the internal pressure can be prevented by using a mold havinga hot runner of the valve gate type, with the nozzle portion beingpressed onto the mold and the valve gate closed.

In order to dissolve gas in molten resin in the injection molding methodof the invention, gas may be supplied into the molten resin portion inthe plasticizing cylinder so as to form a gas space of a predeterminedgas pressure within the plasticizing cylinder in the gas supply section.The amount of the gas dissolved in the molten resin varies with the kindof gas, the kind of resin, the temperature of the molten resin, therotating speed of the screw, the pressure at the front end of the screw,the durations of time for which the screw rotates and remains at rest,etc. but if these conditions are constant, the pressure of the gas incontact with the molten resin is substantially proportional to theamount of the gas dissolved in the molten resin. Therefore, the amountof gas dissolution can be controlled with good reproducibility byforming a gas space in contact with the molten resin and keeping the gaspressure in this gas space at a predetermined level.

In order to ensure that the above-mentioned gas space is formedpositively, the molten resin is preferably transferred in a starvedstate through the gas supply section. By the expression “the moltenresin is transferred in a starved state”, we mean that the molten resinbeing transferred does not completely fill the plasticizing cylinder butleaves a partial empty space. In a preferred case, a plasticizingcylinder of a 2-stage type screw is used, with the first stage of thescrew comprising, from the rear end side (hopper side), a feed section,a compression section and a metering section, and the second stage ofthe screw having the same construction, and the resin is melted in thefirst stage of the screw whereas the feed section of the second stage isused as the gas supply section through which the molten resin istransferred in a starved state, thereby creating an empty space in theplasticizing cylinder and gas is supplied into this empty space so thatit is kneaded with the resin in the second stage of the screw anddissolved in the resin.

The most convenient way to ensure that the molten resin is transferredin a starved state through the gas supply section is by providing a flowcontrol section presenting high resistance to the molten resin in thearea between the metering section of the first stage of the screw andthe vent portion of the second stage of the screw, thereby limiting theingress of the molten resin into the second stage of the screw. Meansfor providing the flow control section that presents high resistance tothe molten resin include decreasing the channel depth of the screw,special designs for kneading such as Dulmage and Madock, and a simplecylindrical barrier having a clearance of only about 0.1-1 mm,preferably 0.1-0.5 mm, from the cylinder.

The above-mentioned flow control section is preferably combined with thepractice of transferring more of the molten resin through the secondstage of the screw than through the first stage per rotation of thescrew. A specific technique may be by adopting greater channel pitchesand depths in the second stage of the screw than in the first stage. Inthis case, the feed section of the second stage (the second feedsection) may have a multi-flight (e.g. double flight) screw to reducethe distance between flights; as a result, the pitch can be set at asufficiently large value to increase the amount of resin transfer; inaddition, even if the molten resin is transferred in a starved state,the occurrence of its retention in the screw channels is seldom enoughto prevent the resin from being deteriorated due to its retention.Another technique that can be combined is by equipping the hopper with adevice for metering the resin supply and creating a starved state bycontrolling the amount of resin supply into the first stage of thescrew.

In addition to the above-described injection of gas in a starved state,the molten resin is preferably kneaded with the gas as part of it isallowed to flow backward on the condition that the starved state ismaintained within the second feed section. By allowing part of themolten resin to flow backward, the gas injected into the molten resincan be brought into contact with a sufficiently increased area of themolten resin to promote its dissolution in the molten resin. Inaddition, mixing of the portions of molten resin before and after gasinjection is sufficiently promoted to suppress uneven dissolution of thegas in the feed direction of the molten resin.

Part of the molten resin can be caused to flow backward in the abovesecond feed section by forming notches in the screw flights in thesecond feed section. By ensuring that the screw in the second feedsection provides a sufficiently large transport, the above-mentionedstarved state can be maintained despite this back-flow of the moltenresin. A kneading effect comparable to that of the back-flow of part ofthe molten resin in the second feed section can also be obtained by, forexample, erecting kneading pins in the second feed section.

If notches for causing part of the molten resin to flow backward are tobe provided in the screw flights in the second feed section to promotethe dissolution of a gas in the molten resin, the total area of thenotches preferably accounts for 1/50-½ of the total area of the screwflights in the second feed section, with those notches being distributedover the screw flights in the second feed section. Note that the area ofeach notch and that of each screw flight are the area of projection inthe direction of the central axis of the screw.

When the pressure gradient of the resin in the first stage of the screwdecreases as when the screw stops rotating, the gas supplied in the gassupply section may pass through the resin in the first stage of thescrew to blow out into the hopper and the flow control section is alsoeffective in preventing this phenomenon. In order to prevent gas blowoutinto the hopper, a moving back-flow preventing mechanism of the sametype as commonly used in screw heads can be employed and this may wellbe called a more positive way. Since the function of the back-flowpreventing mechanism is to prevent the blowout of gas into the hopper bysealing resin of a comparatively low pressure, it does not need to havea rugged construction and a check ball, a moving ring, a lead valve or ashutter in the form of a spur gear with a limited range of rotationangles can be used; a preferred design is such that as soon as theplasticization process ends, the back-flow preventing mechanism isactuated automatically as by a spring or reverse rotation of the screw.

While the foregoing description assumes the 2-stage type screw, screwsconsisting of three or more stages may be used to achieve the sameeffect if the feed section of the front end side screw stage is used asthe gas supply section and the flow control section is provided in themetering section of the subsequent screw stage so that gas can besupplied in an empty space created in the gas supply section. Connectinga plurality of 1-stage screw extruders is also equivalent to an extruderhaving a screw of 2 or more stages.

In order for the injection molding method of the invention to have themolten resin transferred in a consistently starved state through the gassupply section so that gas is dissolved homogeneously in the moltenresin, it is necessary that the pressure at the front end of the screwis at least equal to the gas pressure in the aforementioned gas spaceand within a range where the gas space can be maintained within theplasticizing cylinder in the gas supply section. If the pressure at thefront end of the screw is unduly low, the gas supplied into the gassupply section may not be completely dissolved in the molten resin butmay partly remain in bubble form so that metering of the molten resinbecomes variable or voids occur in the molding; in the latter case, thegas within the voids will expand when the mold is opened and this inturn expands the molding. The pressure at the front end of the screw isthe pressure of the molten resin at the front end of the screw and thisis equal to the back pressure of the screw which pushes the screw or theinjection plunger in the direction of injection.

To set the lower limit of the pressure at the front end of the screw,the following two methods may be employed. One of them is a convenientmethod in which the gas pressure within the gas space in the gas supplysection is set to be equal to the pressure at the front end of thescrew. In the other method, the lower limit is associated with theplasticization of resin and set to a pressure where the rotation of thescrew is proportional to the amount of the plasticized resin; in otherwords, a minimum pressure at which the screw or the injection plungerretracts at a constant speed given a constant rotating speed of thescrew is set as the lower limit.

The first method can be easily effected by providing the plasticizingcylinder with a pressure sensor which detects the gas pressure withinthe gas space formed in the gas supply section and by also providing acontrol unit which controls the pressure at the front end of the screwon the basis of the pressure detected with this pressure sensor. In thiscase, the pressure at the front end of the screw can be controlled tobecome either equal to the pressure in the gas space or greater than itby a predetermined amount. The second method is based on the fact thatwhen the gas supplied into the gas supply section partly mixes with themolten resin in bubble form, the apparent amount of molten resin beingfed by the screw, namely, the sum of the volumes of the molten resin andthe bubbles, increases temporarily, causing the retracting speed of thescrew or the injection plunger to increase abruptly.

If the pressure at the front end of the screw is unduly high, theplasticization of resin slows down and, what is more, if theaforementioned 2-stage type screw is used, the molten resin cannot befed through the second stage of the screw against the high pressure atthe front end of the screw and the pressure of the molten resin in thegas supply section increases so much that no gas space can exist. Insuch a state, it is not only impossible to dissolve an adequate amountof gas in the molten resin but it is also difficult to control theamount of gas being dissolved in the molten resin. The upper limit ofthe pressure at the front end of the screw is a pressure at which whenthe resin is plasticized without gas supply, the pressure of the moltenresin in the gas supply section becomes equal to the gas supplypressure. If the pressure of the molten resin at the front end of thescrew is less than this upper limit, a gas space can exist in the gassupply section and the necessary amount of gas can be dissolved in themolten resin while controlling the amount of its dissolution.

However, in a situation where the gas supply section is filled with asuper atmospheric molten resin in the absence of gas supply, a gas spacecan be formed by supplying the gas supply section with a gas having ahigher pressure than the molten resin but if the pressure of the moltenresin varies, the pressure in the gas space also changes and the amountof the gas dissolved in the molten resin will eventually vary. To avoidthis problem, it is preferred that the transfer of the molten resinthrough the gas supply section is always held in a starved state,irrespective of gas supply, thereby ensuring that the pressure of themolten resin in the gas supply section is kept subatmospheric. In thiscase, the gas space in the gas supply section will not be compressed bythe molten resin and the amount of gas to be dissolved in the moltenresin can be controlled by the pressure of gas being supplied into thegas supply section.

In the present invention, the absolute value of the pressure at thefront end of the screw (back pressure of the screw) varies with the sizeof screw, its design, its rotating speed, the kind of resin, itstemperature, the pressure of gas to be supplied, etc. but it is at leastequal to the pressure of gas to be supplied into the gas supply sectionand selected within a range where the pressure of the molten resin inthe gas supply section is less than the pressure of gas being suppliedinto the gas supply section.

The molten resin with dissolved gas has a tendency to expand, so inorder to suppress the occurrence of foaming during shutdown of the screwafter the end of plasticization until injection starts, theabove-described pressure at the front end of the screw is desirablyretained. The pressure to be retained at the front end of the screw maybe equal to the pressure at the front end of the screw that is appliedduring resin plasticization and may be a minimum pressure at which thescrew or plunger will not retract upon expansion of the molten resin. Ifthe pressure at the front end of the screw which is to be retained afterthe end of plasticization until injection starts is unduly high, thescrew or plunger will have advanced before the injection process starts,causing an error in the amount of molten resin that was metered duringplasticization. Even in the case of suspending the molding process for ashort time, the pressure at the front end of the screw is preferablyretained at a non-screw or plunger retracting level in order to preventfoaming of the residual molten resin in the plasticizing cylinder.

A technique by which the pressure at the front end of the screw can bemaintained constant during shutdown of the screw is one of maintaining aconstant force for advancing the screw and in the case where the screwis advanced by hydraulic drive, this can be accomplished by maintainingthe screw advancing hydraulic force constant. In this case of hydraulicdrive, the desired effect can also be obtained by closing a screwadvancing hydraulic valve during screw shutdown to cut the flow of aworking oil countering the pressure at the front end of the screw, thuslocking the screw against retraction.

Control may also be effected to maintain the position of the screw orplunger constant and this is suitable for the case where the screw isadvanced by drive with an electric motor.

The supply of gas into the plasticizing cylinder in the gas supplysection is preferably effected via an automatic on-off valve that isopened and closed automatically by the pressure difference between thesupply pressure and the pressure within the cylinder in the gas supplysection.

Specific examples of such automatic on-off valve include a mushroomvalve (a valve body in the form of a frustum, typically a frustum of acone) that is provided at the distal end of a gas supply channel open tothe interior of the plasticizing cylinder in the gas supply section suchthat it is urged by a spring in a direction where it makes intimatecontact with the valve seat and which, when pushed by the pressure ofgas being supplied, counters the spring and moves into the plasticizingcylinder to open the gas supply channel. If such a mushroom valve isused, the gas supply channel is opened in a gas supply mode inaccordance with the gas flow to provide a comparatively large area ofopening and, hence, the necessary amount of gas can be supplied within ashort time. On the other hand, when there is no gas flow, the gas supplychannel is closed automatically, so the molten resin will not flowbackward into the gas supply channel. Furthermore, even in the casewhere gas is flowing, any part of the molten resin that is about to getinto the gas supply channel will push the bottom of the mushroom valveuntil it becomes closed automatically, whereby the molten resin can beprevented from flowing backward.

In the next place, an example of the apparatus to be used for theinjection molding method of the invention is described with reference tothe accompanying drawings. The apparatus of this example uses an in-linescrew type injection molding machine and carbon dioxide as a gas.

In FIG. 1, numeral 1 designates an injection molding machine which isfurnished with a plasticizing cylinder 2 which plasticates and injects athermoplastic resin, a mold 3 and a mold clamping device 4. Theplasticizing cylinder 2 in the injection molding machine 1 is suppliedwith carbon dioxide from a carbon dioxide source 5 via a gas supplydevice comprising a carbon dioxide booster 6 and a carbon dioxidepressure control 7.

If desired, carbon dioxide may be supplied into the mold 3 and used as agas to produce counterpressure; alternatively, it may be supplied into ahopper 8 on the plasticizing cylinder 2 so that it is absorbed by aresin being supplied from the hopper 8 into the plasticizing cylinder 2.In this case, the carbon dioxide to be supplied into the plasticizingcylinder 2, the carbon dioxide to be supplied into the mold 3 and thecarbon dioxide to be supplied into the hopper 8 are preferably adaptedto be controllable independently in such terms as the start and stop ofsupply and the supply pressure.

The above plasticizing cylinder 2, carbon dioxide source 5, carbondioxide booster 6 and carbon dioxide pressure control 7 are furtherdescribed with reference to FIG. 2.

As shown in FIG. 2, a liquefied carbon dioxide container 9 is used asthe carbon dioxide source 5 in the case under consideration.

The carbon dioxide booster 6 is equipped with a liquefied carbon dioxidecompressor 11 which pressurizes liquefied carbon dioxide to have anelevated pressure and the liquefied carbon dioxide container 9 isconnected to the liquefied carbon dioxide compressor 11 via anelectromagnetic on-off valve 10. The space between the carbon dioxidesource 5 and the carbon dioxide booster 6 is held below the criticaltemperature of carbon dioxide (31.1° C.) in order to keep the carbondioxide liquefied. The liquefied carbon dioxide supplied into theliquefied carbon dioxide compressor 11 from the liquefied carbon dioxidecontainer 9 and which has been compressed there to have a higherpressure is sent to the carbon dioxide pressure control 7.

The liquefied carbon dioxide sent to the carbon dioxide pressure control7 is supplied to a heater 13 via an electromagnetic on-off valve 12. Theliquefied carbon dioxide supplied into the heater 13 is heated there tobecome a gas hotter than the critical temperature and the gas passesthrough a reducing valve 14 to be supplied into a main tank 15 for theplasticizing cylinder 2. The main tank 15 is connected to a relief valve16 for escape of gas if the internal pressure becomes abnormally highand a meter 17 for checking the gas pressure in the main tank 15.

The above main tank 15 and the plasticizing cylinder 2 are connected bya gas supply pipe 18 which are equipped, in order from the main tank 15,with an electromagnetic on-off valve 19 and a check valve 20. Connectedbetween the check valve 20 and the plasticizing cylinder 2 are a reliefvalve 21 and a valve 22 open to the atmosphere.

We now describe the procedure of operating the above-described gassupply device to supply carbon dioxide into a gas supply section 26 inthe plasticizing cylinder 2.

First, the electromagnetic on-off valve 10 is opened with theelectromagnetic on-off valves 19 and 20 closed, whereupon liquefiedcarbon dioxide is supplied from the liquefied carbon dioxide container 9into the liquefied carbon dioxide compressor 11. Upon opening theelectromagnetic on-off valve 12, the liquefied carbon dioxide gascompressed in the liquefied carbon dioxide compressor 11 is suppliedinto the heater 13 and warmed, then its pressure is reduced to thenecessary level by the reducing valve 12 before it is stored in the maintank 15. After the pressurized gas with the necessary pressure has beenstored in the main tank 15, the electromagnetic on-off valve 19 isopened and a predetermined pressure of carbon dioxide is supplied intothe plasticizing cylinder 2 via the gas supply pipe 18.

The plasticizing cylinder 2 is equipped with a 2-stage type screw 23 inwhich a stage comprising, in order from the resin supply section towardthe front end, a feed section, a compression section and a meteringsection is repeated twice in series and the first stage of the screw 23a is closer to the hopper 8 and the second stage of the screw 23 b iscloser to the nozzle portion 24. As shown in FIG. 7, the first stage ofthe screw 23 a comprises a first feed section 41, a first compressionsection 42 and a first metering section 43 and similarly the secondstage of the screw 23 b comprises a second feed section 44, a secondcompression section 45 and a second metering section 46.

A resin metering unit 25 is connected to the hopper 8 on theplasticizing cylinder 2 and a metered and controlled quantity of resinis supplied into the hopper 8. By connecting the resin metering unit 25to the hopper 8, there is provided another advantage in that the amountof resin supply is controlled to provide greater ease with which themolten resin can be transferred in a starved state through the gassupply section 26.

The gas supply section 26 is located in the feed section (vent portion)of the second stage of the screw 23 b and a gas supply channel 27 isopen to this gas supply section 26. The gas supply channel 27 isconnected to the aforementioned gas supply pipe 18.

A flow control section 28 is provided between the gas supply section 26and the first stage of the screw 23 a. FIG. 3 shows enlarged an exampleof the flow control section. The flow control section 28 has a smallclearance t from the inner surface of the barrel of the plasticizingcylinder 2 and controls the amount of the molten resin being transferredfrom the first stage of the screw 23 a to ensure that the molten resinis transferred in a starved state through the gas supply section 26 andthat the carbon dioxide supplied into the gas supply section 26 will notflow backward to the hopper 8. If a mechanism for preventing theback-flow of the molten resin is to be provided, it is installed betweenthe flow control section 28 and the gas supply section 26 (see aback-flow preventing mechanism 40 in FIG. 2) and, depending on the screwdesign, the flow control section 28 may be omitted.

The clearance t between the flow control section 28 and the innersurface of the barrel of the plasticizing cylinder 2 is variable withthe screw diameter but it is preferably about 0.1-1 mm, more preferablyabout 0.1-0.5 mm. The length 1 of the flow control section 28 ispreferably about 5-200% of the screw diameter, more preferably about10-100% of the screw diameter.

The clearance t and the length 1 are selected as appropriate for themelt viscosity of resin and the pressure of gas to be supplied. Thelower the melt viscosity of the resin used and the higher the pressureof carbon dioxide to be supplied into the gas supply section 26, thesmaller the clearance t and the greater the length 1. By adjusting thesevalues, if the first stage of the screw 23 a is filled with the resin,the carbon dioxide in the gas supply section 26 can be positivelyprevented from flowing backward to the hopper 8 during the moldingoperation. If desired, the temperature of the molten resin passingthrough the flow control section 28 may be lowered to enhance itsviscosity and this is also effective in ensuring that the carbon dioxidesupplied into the gas supply section 26 will not flow backward to thehopper 8.

As shown in FIG. 7 and in FIGS. 8A and 8B, the screw flights in thesecond feed section of the screw 23 are provided with notches 50 forcausing part of the molten resin to flow backward on the condition thata starved state is maintained in the second feed section, therebypromoting dissolution of the gas being supplied from the gas injectionchannel 27 into the molten resin. As the molten resin is supplied fromthe first stage of the screw 23 to the second feed section, it is fedfast and forward through the second feed section on account of the deepscrew flights and the double flights with large pitch but, at the sametime, part of the molten resin flows backward via the notches 50,thereby effecting adequate kneading. As a result, gas such as carbondioxide that has been supplied from the gas injection channel 27 intothe second feed section is dispersed more uniformly and its dissolutioninto the molten resin is promoted.

The shape of the above notches 50 is semicircular in the case underconsideration but they may be triangular or rectangular. As alreadymentioned, the total area of the notches 50 is preferably from 1/50 to ½of the total area of the screw flights in the second feed section and amore preferred range is from 1/20 to ⅓. It is also preferred that thenotches 50 are distributed over all of the screw flights in the secondfeed section. In the case under consideration, the notches 50 areprovided equidistantly in the helical direction of the screw flights.

To supply carbon dioxide into the gas supply section 26, the moltenresin is transferred in a starved state through the gas supply section26 by means of the above flow control section 28 and the empty spacethus formed in the gas supply section 26 (the area not filled with themolten resin) is supplied with carbon dioxide via an automatic on-offvalve 29 shown in FIG. 4.

As shown in FIG. 4, the automatic on-off valve 29 in the case underconsideration comprises a mushroom valve which is urged outward of theinjection cylinder 2 by means of a spring 30 so that it is pressed ontoa valve seat 31.

To further explain, the above automatic on-off valve 29 is positioned atthe distal end of the gas supply channel 27 and on the peripheral edgeof the distal end of the gas supply channel 27, there is provided avalve seat 31 facing the back of the automatic on-off valve 29. A valveshaft 32 is connected to the back of the automatic on-off valve 29; thisvalve shaft 32 extends through the gas supply channel 27 with aclearance provided around it and it is urged outward of the plasticizingcylinder 2 by means of the spring 30.

When the pressure in the gas supply channel 27 is equal to the pressurein the gas supply section 26 of the plasticizing cylinder 2, the aboveautomatic on-off valve 29 is pressed onto the valve seat 31 by the forceof the spring 30 to close the gas supply channel 27; when carbon dioxideis supplied into the gas supply channel 27 until the pressure in the gassupply channel 27 becomes higher than the pressure in the gas supplysection 26 of the plasticizing cylinder 2 so that the force exerted bythe pressure of carbon dioxide exceeds the force of the spring 30, theautomatic on-off valve 29 moves into the plasticizing cylinder 2 againstthe spring 30 to open the gas supply channel 27.

If the automatic on-off valve 29 using the above mushroom valve isemployed to supply carbon dioxide, the gas supply channel 27 opens in acarbon dioxide supply mode in accordance with the flow of carbon dioxideto provide a comparatively large area of opening and, hence, thenecessary amount of carbon dioxide can be supplied within a short time.On the other hand, when there is no flow of carbon dioxide, the gassupply channel 27 is closed automatically, so the molten resin will notflow backward into the gas supply channel 27. Furthermore, even in thecase where carbon dioxide flows, any part of the molten resin that isabout to get into the gas supply channel 27 will push the bottom of theautomatic on-off valve 29 to close the gas supply channel 27automatically, whereby the molten resin can be prevented from flowingbackward.

In order to prevent the molten resin from convecting in the neighborhoodof its distal end face (the face on the inside of the plasticizingcylinder 2), the above automatic on-off valve 29 is preferablypositioned such that when it is open, its distal end face issubstantially flush with the inner surface of the plasticizing cylinder2. Specifically, the automatic on-off valve 29 is preferably such thatwhen it is closed, the position of its distal end face is substantiallyequal to or recessed about 0.5 mm from the position of the inner surfaceof the plasticizing cylinder 2.

As shown in FIG. 2, the front end portion of the screw 23 is providedwith a back-flow preventing mechanism 33 for preventing the molten resinfrom flowing backward when it is being injected. The back-flowpreventing mechanism 33 may be as shown in FIG. 9; a small-diameterportion 60 is formed in the front end portion of the screw 23 and anannular check ring 62 is inserted into the small-diameter portion 60,with a resin flow channel 61 left between the ring 62 and the outerperipheral surface of the small-diameter portion 60, in such a way thatthe ring 62 is capable of moving along the central axis of the screw 23,and a spring 63 is provided to press the check ring 62 toward the rearend (toward the hopper 8). The back-flow preventing mechanism 33operates as follows: when the screw 23 is at rest, the pushing force ofthe spring 63 pushes the check ring 62 to move toward the rear end toclose the resin flow channel; when the screw 23 rotates in forwarddirection in the presence of the molten resin, the advancing force ofthe molten resin pushes the check ring 62 to move toward the front endagainst the spring 63, thereby opening the resin flow channel. Thenozzle portion 24 is also provided with a needle valve 35 for opening orclosing a nozzle hole 34. This needle valve 35 is provided within thenozzle portion 24 in such a way that it can move toward or away from thenozzle hole 34. When a drive rod 37 is tilted by a drive unit 36 such asa hydraulic cylinder, the needle valve 35 is moved back and forth and itcloses the nozzle hole 34 when it advances whereas it opens the nozzlehole 34 when it retracts.

Providing the needle valve 35 of the above type to ensure that thenozzle hole 34 can be opened and closed offers the advantage that ifplasticizing and metering operations are performed by exerting pressure(back pressure of the screw) on the front end portion of the screw 23with the nozzle hole 34 kept closed, the molten resin with dissolvedcarbon dioxide that collects in the front end portion of theplasticizing cylinder 2 after metering can be prevented from expanding.

In the injection molding method of the invention, it is preferred to usethe plasticizing cylinder 2 which is capable of opening and closing thenozzle hole 34; the mechanism for opening and closing the nozzle hole 34is not limited to the above type which forces it to open and closemechanically and there may be used such a type that the nozzle hole 34opens automatically when the pressure of the molten resin in theplasticizing cylinder 2, particularly the pressure of the molten resinat its front end, has reached a predetermined level. In other words, theinjection molding machine to be used in the invention preferably employsthe plasticizing cylinder 2 equipped with a valve nozzle that can beopened or closed.

While the above-described apparatus uses carbon dioxide as a gas, theinjection molding method of the invention can be performed with the sameapparatus even if gases other than carbon dioxide are used.

EXAMPLES

The following examples and comparative examples are provided for thepurpose of further illustrating the present invention.

To begin with, the materials, the equipment and the method for measuringthe quantity of carbon dioxide in molten resin that were used in theexamples and comparative examples are described.

Resins

Rubber-reinforced polystyrene: A&M POLYSTYRENE-492, product of A&MStyrene Co.

Poly(methyl methacrylate) resin (PMMA): DELPET 70NHX, product of ASAHIKASEI CORP.

Carbon Dioxide

Carbon dioxide with a purity of at least 99% was used.

Injection Molding Machine

SG125M-HP of SUMITOMO HEAVY INDUSTRIES, LTD. was used as a base. Thismolding machine was of an in-line screw type and the screw was of a2-stage type with L/P=23; the screw design had the construction shown inFIG. 2. The screw diameter was 32 mm; the depth of screw flights was 3.8mm in the first feed section, 1.7 mm in the first metering section, 7 mmin the second feed section, and 1.9 mm in the second metering section.The second feed section had double flights. Two types of screw wereused, one containing notches and the other containing no notches. Thenotches were semicircular with a radius of 3 mm and four notches wereprovided equidistantly per rotation. A cylindrical flow control section20 mm wide was provided in the first stage of the screw and theclearance from the inner surface of the cylinder was adjusted to 0.25mm. The gas supply section had the structure shown in FIG. 3 and amushroom valve of the type shown in FIG. 4 was installed as an automaticon-off valve. The part of the mushroom valve which would contact thevalve seat had a diameter of 4.6 mm and this mushroom valve was closedwith a spring by a force of 300 g. The nozzle portion was designed tohave the mechanical opening/closing mechanism shown in FIG. 2. Althoughnot shown, a pressure sensor for measuring the pressure of molten resinor gas was attached to the inner surface of the cylinder on the sideopposite the side where the gas supply channel was open to the gassupply section. NP465XL of DAINISCO Co., Ltd. was used as the pressuresensor.

Setting the Injection Cylinder Temperature

The temperature settings of the injection cylinder during injectionmolding were 200 and 210° C. in the case where the resin used wasrubber-reinforced polystyrene and 240° C. in the case of a poly(methylmethacrylate) resin.

Measuring the Amount of Carbon Dioxide in Molten Resins

The amount of carbon dioxide dissolved in molten resin was measured fromthe decrease in the weight of a molded article after molding.Specifically, the weight of a molded article was measured immediatelyafter molding; thereafter, the molded article was left to stand in theatmosphere for about 24 hours, then left to stand in a vacuum dryer at80° C. for 48 hours and another weight measurement was made; thedifference between the weights measured before and after standing wasused as the amount of carbon dioxide contained in the molten resin.

Mold

A mold for shaping a rectangular flat plate 2 mm thick, 120 mm long and60 mm wide was used.

Reference Example 1

A screw containing notches in the second feed section and a rubberreinforced polystyrene resin were used; the temperature of theplasticizing cylinder was set at 210° C. and molding was performed withthe screw rotating at 150 rpm and with a molding cycle time of 40seconds.

Using the gas supply device shown in FIG. 2, the pressure of carbondioxide to be supplied into the gas supply section of the plasticizingcylinder was varied and injection molding was performed with thepressure at the front end of the screw being made equal to the pressureof carbon dioxide being supplied into the vent portion, and the amountof carbon dioxide in the molten resin was measured. The result is shownin FIG. 5.

As FIG. 5 shows, the pressure of carbon dioxide supplied to the gassupply section is proportional to the amount of carbon dioxide dissolvedin the molten resin and it is seen that the amount of carbon dioxidedissolved in the molten resin can be controlled by the pressure ofcarbon dioxide.

Reference Example 2

Without supplying gas but with the nozzle hole closed, the actions ofplasticizing and purging a resin were repeated with a screw stroke of 50mm.

While the screw was rotating at 50, 100 or 150 rpm, the pressure at itsfront end was varied and the pressure of the molten resin that wasexerted on the gas supply section by each of the pressures at the frontend of the screw was measured with the pressure sensor and the result isshown in FIG. 6.

As FIG. 6 shows, if the pressure at the front end of the screw isincreased with the rpm of the screw held constant, the pressure of themolten resin comes to be detected in the gas supply section when thepressure at the front end of the screw exceeds a certain level. When thepressure at the front end of the screw exceeded 14 MPa while the screwwas rotating at 150 rpm, a molten resin's pressure greater than 0 MPawas detected, indicating that the gas supply section was filled with themolten resin and that it could not be transferred in a starved state.

Example 1

A screw containing notches in the second feed section and a rubberreinforced polystyrene resin were used; the temperature of theplasticizing cylinder was set at 210° C. and 30 purging actions wereperformed with the screw rotating at 150 rpm, with a pressure of 10 MPabeing exerted at the front end of the screw and with 10 MPa of carbondioxide being supplied into the gas supply section. The strand from thenozzle portion foamed finely but it did not break nor was heard anysound of bursting bubbles; it was therefore speculated that no largebubbles existed in the molten resin.

Subsequently, a mold with a surface temperature of 40° C. the cavity ofwhich had been filled with 8 MPa of carbon dioxide was filled with theresin in 0.6 seconds, subjected to dwelling for 3 seconds with thepressure of the molten resin in the plasticizing cylinder maintained at110 MPa, and cooled for 20 seconds. The cavity filling carbon dioxidewas released to the atmosphere simultaneously with the end of the resinfilling.

Fifty shots were molded with a cycle time of 40 seconds and yet theplasticization time was 6.5 seconds with a fluctuation of no more than0.3 seconds. The moldings experienced no blistering and they had no foampatterns on the surfaces; they had a good appearance with no cells foundin the interior. The amount of carbon dioxide dissolved in the moltenresin as obtained from the decrease in the weight of the moldings was2.3 wt %.

Example 2

A screw containing notches in the second feed section and a rubberreinforced polystyrene resin were used; the temperature of theplasticizing cylinder was set at 210° C. and purging actions wereperformed with the screw rotating at 150 rpm and with a pressure of 18MPa being exerted at the front end of the screw. In this case, when nocarbon dioxide was supplied into the gas supply section, a pressure of0.8 MPa was detected for the molten resin, indicating that the gassupply section was filled with the resin. In this state, 10 MPa ofcarbon dioxide was supplied into the gas supply section and purging andmolding were performed as in Example 1.

In each purging action, the strand from the nozzle portion foamed finelybut it did not break nor was heard any sound of bursting bubbles; it wastherefore speculated that no large bubbles existed in the molten resin.

Fifty shots were molded with a cycle time of 40 seconds and yet theplasticization time was 8.3 seconds with a fluctuation of no more than0.4 seconds. The moldings experienced no blistering and they had no foampatterns on the surfaces; they had a good appearance with no cells foundin the interior. The amount of carbon dioxide dissolved in the moltenresin as obtained from the decrease in the weight of the moldings was2.0 wt %.

Comparative Example 1

A screw containing notches in the second feed section and a rubberreinforced polystyrene resin were used; the temperature of theplasticizing cylinder was set at 210° C. and purging and molding wereperformed as in Example 2, except that the pressure at the front end ofthe screw was adjusted to 9 MPa.

In the purging action, the strand from the nozzle portion foamed finelyand broke; hence, no continuous strand was obtained and sound ofbursting bubbles was heard, indicating that large bubbles existed in themolten resin. The plasticization time was largely variable between about3 and 6 seconds and blistering occasionally occurred in the moldings.

Comparative Example 2

A screw containing notches in the second feed section and a rubberreinforced polystyrene resin were used; the temperature of theplasticizing cylinder was set at 210° C., the screw was rotating at 150rpm and a pressure of 30 MPa being exerted at the front end of thescrew. When no carbon dioxide was supplied into the gas supply section,a pressure of 12 MPa was detected for the molten resin, indicating thatthe gas supply section was filled with the molten resin.

In this state, 10 MPa of carbon dioxide was supplied into the gas supplysection and purging and molding were performed as in Example 2.

In each purging action, the strand from the nozzle portion did not foambut remained clear; when the molten resin was left to stand, slightfoaming occurred to such an extent that a few cells formed in theinterior. This indicated that carbon dioxide hardly dissolved in themolten resin.

Molding was performed without pressurizing the cavity with carbondioxide. The moldings were clear and had no foam patterns on thesurface. The amount of carbon dioxide dissolved in the molten resin asobtained from the decrease in the weight of the moldings was no morethan 0.1 wt %.

Example 3

With the temperature of the injection cylinder set at 210° C., injectionmolding was performed using a rubber reinforced polystyrene. Thepressure of carbon dioxide being supplied to the gas supply section washeld at 9 MPa, the screw was rotating at 150 rpm, a pressure of 11 MPawas exerted at the front end of the screw, and a counterpressureproduced by 7 MPa of carbon dioxide was applied in order to preventfoaming from occurring while the mold was in the process of filling withthe molten resin. Using two types of screw, one containing notches inthe screw flights in the second feed section and the other containing nonotches, molding was performed with the molding cycle time varied overthe range of from 26 to 100 seconds in each case.

From the moldings obtained, the amount of carbon dioxide dissolved inthe molten resin was measured. The result is shown in FIG. 10.

As FIG. 10 shows, when the screw containing notches in the screw flightsin the second feed section was used, the dissolution of carbon dioxideincreased by a factor of 1.5 compared to the case of using the screwcontaining no such notches, indicating that an increased amount ofcarbon oxide could be dissolved. In addition, when the screw containingnotches in the screw flights in the second feed section was used, thefluctuation in the amount of dissolved carbon dioxide due to thefluctuation in the molding cycle time was small compared to the case ofusing the screw containing no such notches, indicating the possibilityof performing more consistent molding.

Example 4

The amount of carbon dioxide dissolved in the molten resin was measuredas in Example 3, except that the resin to be used was changed to PMMA.The experiment was done using a screw containing notches in the screwflights in the second feed section and a screw containing no suchnotches.

The result is shown in FIG. 11. Like FIG. 10, FIG. 11 shows therelationship between the molding cycle time and the amount of carbondioxide dissolved in the molten resin, provided that the vertical axisplots the amount of carbon dioxide for each molding cycle time in termsof ratio, with the amount of carbon dioxide being taken as unity for thecase where the molding cycle time was 100 seconds.

As FIG. 11 shows, when the screw containing notches in the screw flightsin the second feed section was used, more carbon dioxide could bedissolved than in the case of using the screw containing no suchnotches. In addition, when the screw containing notches in the screwflights in the second feed section was used, the fluctuation in theamount of dissolved carbon dioxide due to the fluctuation in the moldingcycle time was small compared to the case of using the screw containingno such notches, indicating the possibility of performing moreconsistent molding.

Industrial Applicability

The present invention is as described on the foregoing pages; thenecessary amount of gas can be dissolved in molten resin with goodreproducibility in a quantitative manner; on account of the notches madein the screw, gas of low enough pressure can be dissolved in a greateramount in the molten resin to perform injection molding; as a result,injection molding which has gas dissolved in the molten resin to improveits flowability, as well as foam molding using a dissolved gas can beperformed efficiently. In addition, the fluctuation in the amount ofdissolved gas that accompanies the fluctuation in the molding cycle timecan be sufficiently reduced to enable consistent injection molding.

1. An injection molding method comprising supplying a gas into a moltenresin in a plasticizing cylinder and injecting the molten resin havingthe gas dissolved therein, wherein when the resin is being plasticized,a gas space with a predetermined gas pressure is formed within theplasticizing cylinder in a gas supply section and the pressure at thefront end of a screw is adjusted to be at least equal to the gaspressure in the gas space and within a range where the gas space can bemaintained within the plasticizing cylinder in the gas supply section,wherein the plasticizing cylinder is equipped with a multi-stage typescrew comprising a front end side screw stage having screw flightswherein notches are formed in at least one of said screw flights and thegas and the molten resin are kneaded to dissolve the gas into the moltenresin while part of the molten resin is caused to flow backward by meansof said notches, and wherein the gas pressure in the gas space is keptat a predetermined level to control the gas dissolution amount.
 2. Theinjection molding method according to claim 1, wherein the gas pressurein the gas space formed in the gas supply section is detected and thepressure at the front end of the screw is controlled on the basis ofthis gas pressure.
 3. The injection molding method according to claim 1,wherein the molten resin is transferred in a starved state within thegas supply section.
 4. The injection molding method according to claim1, wherein the pressure at the front end of the screw exerted duringplasticization is also retained during the screw shutdown period fromthe end of plasticization to the start of injection.
 5. The injectionmolding method according to claim 1, wherein the gas is carbon dioxide.6. The injection molding method according to claim 1, wherein theplasticizing cylinder is equipped with a multi-stage type screw in whicha stage comprising, in the following order from the resin supply sectionside toward the front end side of the screw, a feed section, acompression section and a metering section is repeated a plurality oftimes in series, and the gas supply section is located within the feedsection of the front end side screw stage.
 7. The injection moldingmethod according to claim 6, wherein a flow control section presentinghigh resistance to the flow of the molten resin is provided in themetering section of the rear end side screw stage of the plasticizingcylinder.
 8. The injection molding method according to claim 6, whereinthe plasticizing cylinder is equipped with a mechanism for preventing aback-flow of the molten resin in the metering section of the rear endside screw stage.
 9. The injection molding method according to claim 1,wherein the gas supply into the plasticizing cylinder in the gas supplysection is effected via an automatic on-off valve which is pressed by aspring onto a valve seat provided on the perimeter of an opening in agas supply channel that extends into the plasticizing cylinder andwhich, when gas is supplied into the gas supply channel, is pushedagainst the spring by the pressure difference between the supplypressure and the pressure within the cylinder in the gas supply sectionand moved into the plasticizing cylinder to open the gas supply channel.10. An injection molding apparatus which comprises a plasticizingcylinder and a gas supply device for supplying a gas into a molten resinin the plasticizing cylinder and which injects the molten resin havingthe gas dissolved therein, wherein said plasticizing cylinder has amulti-stage type screw in which a stage comprising, in the followingorder from the rear end side toward the front end side in the directionof injection, a feed section, a compression section and a meteringsection is repeated a plurality of times in series, and which is alsoequipped with a gas supply channel open to the feed section of the frontend side screw stage, wherein said gas supply device is connected to thegas supply channel, and wherein notches are in at least one screw flightin the feed section of the front end side screw stage.
 11. The injectionmolding apparatus according to claim 10, further comprising a pressuresensor for detecting the pressure of the gas supplied into theplasticizing cylinder via the gas supply channel and a control unit forcontrolling the pressure at the front end of the screw on the basis ofthe pressure as detected with said sensor.
 12. The injection moldingapparatus according to claim 10, further comprising an automatic on-offvalve, which is pressed by a spring onto a valve seat provided on theperimeter of an opening in the gas supply channel that extends into theplasticizing cylinder and which, when gas is supplied into the gassupply channel, is pushed against the spring by the pressure differencebetween the supply pressure and the pressure within the cylinder in thegas supply section and moved into the plasticizing cylinder to open thegas supply channel.
 13. The injection molding apparatus according toclaim 10, wherein a flow control section presenting high resistance tothe flow of the molten resin is provided in the metering section of therear end side screw stage.
 14. The injection molding method according toclaim 1, wherein the total area of the notches accounts for 1/50-½ ofthe total area of the screw flights, wherein the area of each notch andthat of each screw flight are based on the area of projection in thedirection of the central axis of the screw.
 15. The injection moldingapparatus according to claim 10, wherein the total area of the notchesaccounts for 1/50-½ of the total area of the screw flights, wherein thearea of each notch and that of each screw flight are based on the areaof projection in the direction of the central axis of the screw.